(1) Field of the Invention
This invention relates to an electrical heating element material of a sheet form which comprises a synthetic resin matrix having metal conductor wires arranged therein and to a process for preparing the electrical heating element material. It also relates to a heat-shrinkable synthetic resin cover including this electrical heating element material therein.
(2) Description of the Prior Art
Tubular or flat sheet-shaped heat-shrinkable synthetic resin covers have heretofore been used as anti-corrosive coverings of joints of steel pipes for pipelines or as protective coverings of joints of electric cables. More specifically, a tubular heat-shrinkable cover is placed around the joint, or a flat sheet-shaped heat-shrinkable cover is wound around the joint and the ends thereof are joined together, and the heat-shrinkable cover is shrunk under heating from an appropriate external heat source to form a coating tightly covering the joint. According to the conventional method, shrinkage under heating is accomplished by direct contact with flames while using a gas burner or the like. In this heat-shrinking method, great technical skill is required for formation of coatings, and under violent climatic conditions, for example, when it is extremely cold or there is a strong wind, this heat-shrinking operation is very troublesome and difficult, and it is impossible to attain a substantially uniform and sufficient heating effect. Therefore, it is impossible to form a sufficiently anti-corrosive or protective coating or covering on the above-mentioned joint.
As means for moderating the above-mentioned defects of the conventional heat-shrinkable covers of the external heating type, there can be considered a method in which an electrical heating element is built in the interior of a heat-shrinkable cover. In this heat-shrinkable cover, when it is located around the joint and shrunk under heating, since external heating using a heat source such as a burner or a heat ray generator need not be carried out, it is expected that heat-shrinking of this cover can be accomplished irrespectively of climate conditions.
In the heat-shrinkable cover including the electrical heating element, this heating element is required to heat the entire cover uniformly while not substantially degrading the heat shrinkability of the cover. Furthermore, this heating element should be manufactured with ease on an industrial scale, and it should be possible to place the heating element in the cover simply and easily to facilitate the manufacture of the heat-shrinkable cover.